Corrosion is a naturally occurring phenomenon commonly defined as the deterioration of a substance (usually a metal) or its properties as a result of a reaction with its environment. Like other natural hazards such as earthquakes or severe weather disturbances, corrosion can cause dangerous and expensive damage to everything from vehicles and home appliances to wastewater systems, pipelines, bridges, roadways and public buildings. Unlike weather-related disasters, however, there are time-proven methods to prevent and control corrosion that can reduce or eliminate its impact on public safety, the economy, and the environment.
The 2001 U.S. Federal Highway Administration-funded cost of corrosion study, “Corrosion Costs and Preventive Strategies in the United States,” determined the annual direct cost of corrosion to be a staggering $276 billion. The study covered a large number of economic sectors, including the transportation infrastructure, electric power industry, conveyance and storage.
The indirect cost of corrosion was conservatively estimated to be equal to the direct cost, giving a total direct plus indirect cost of more than $600 billion or 6 percent of GDP. This cost is considered to be a conservative estimate since only well-documented costs were used in the study. In addition to causing severe damage and threats to public safety, corrosion disrupts operations and requires extensive repair and replacement of failed assets.
The U.S. Federal Highway Administration has rated almost 200,000 bridges, or one of every three bridges in the U.S., as structurally deficient or functionally obsolete. Furthermore, more than one-fourth of all bridges are over 50 years old, the average design-life of a bridge.
The road and bridge infrastructure in the United States is crumbling, with thousands of bridges rated as unsafe and in need of replacement or major repairs. In many of these cases, corrosion plays a significant role in undermining safety. Corrosion protection measures could help minimize or avoid further problems. Steps are being taken to address America's aging infrastructure. For example, House bill H.R. 1682, the “Bridge Life Extension Act 2009,” introduced in March 2009, would require States to submit a plan for the prevention and mitigation of damage caused by corrosion when seeking federal funds to build a new bridge or rehabilitate an existing bridge.
Many reinforced concrete structures suffer from premature degradation. Concrete embedded steel reinforcement is initially protected from corrosion by the development of a stable oxide film on its surface. This film, or passivation layer, is formed by a chemical reaction between the highly alkaline concrete pore water and the steel. The passivity provided by the alkaline conditions may be destroyed by the presence of chloride. The chloride ions locally de-passivate the metal and promote active metal dissolution. Corrosion of the steel is usually negligible until the chloride ions reach a concentration where corrosion initiates. The threshold concentration depends on a number of factors including, for example, the steel microenvironment, the pore solution pH, the interference from other ions in the pore solution, the electrical potential of the reinforcing steel, the oxygen concentration and ionic mobility. The chloride acts as a catalyst in that it does not get consumed in the corrosion reaction but remains active to again participate in the corrosion reaction.
Damage to reinforced concrete structures is caused primarily by the permeation of chloride ions through the concrete to the area surrounding the steel reinforcement. There are a number of sources of chlorides including additions to the concrete mix, such as chloride-containing accelerating admixtures. The chloride may also be present in the structure's environment such as marine conditions or de-icing salts. The presence of chloride does not have a directly adverse effect on the concrete itself, but does promote corrosion of the steel reinforcement. The corrosion products that form on the steel reinforcement occupy more space than the steel reinforcement causing pressure to be exerted on the concrete from within. This internal pressure builds over time and eventually leads to cracking and spalling of the concrete. Corrosion of the steel reinforcement also reduces the strength of the reinforcing steel and diminishes the load bearing capacity of the concrete structure.
Other factors besides chloride ion concentration affect the corrosion rate of steel, including pH, oxygen availability, and electrical potential of the steel, as well as resistivity of the surrounding concrete. These factors interact, such that a limitation on one does not necessarily prevent corrosion and levels approaching threshold levels of one will synergize with another to allow corrosion. For example, even with a high chloride level if insufficient oxygen is available, corrosion will not occur. As the pH falls, the chloride threshold for corrosion becomes lower. In very high resistivity concrete, not only does carbonation and chloride ingress slow, the corrosion reaction is reduced due to the increased difficulty of ion flow. Temperature is also involved in corrosion activity, just like any other chemical reaction.
Cathodic protection of steel reinforcement in concrete is an accepted method of providing corrosion protection for the metal, especially where chloride ions are present at significant concentrations in the concrete. Cathodic protection involves the formation of a circuit with the reinforcing steel acting as a cathode that is electrically connected to an anode. When a sufficiently large potential difference exists, corrosion of the cathode is reduced or prevented.
It is known to create a potential difference between an anode and a cathode both by means of impressed current cathodic protection and by means of a galvanic cell. Impressed current cathodic protection involves the use of an anode and an applied electrical current employing an external DC power supply or an AC power source and a rectifier. The power supply presents challenges in terms of reliability and costs associated with ongoing power consumption, monitoring, control, and maintenance requirements.
Control of the current for impressed current cathodic protection systems is a huge challenge. The amount of energy supplied, whether constant current or voltage ICCP, changes as the temperature, moisture content, chloride exposure, and pH change and must be adjusted through different zones to prevent overprotection (hydrogen embrittlement, acid formation, etc . . . ) or underprotection (corrosion).
Cathodic protection may also be provided by means of a galvanic cell in which the potential arises as a result of different materials forming a sacrificial anode and a cathode. Sacrificial cathodic protection occurs when a metal is coupled to a more reactive, or more anodic, metal. The anode consists of a sacrificial metal that is capable of providing protective current without the use of a power supply, since the reactions that take place during their use are thermodynamically favored. Disadvantages of sacrificial anode systems include limited available protection current and limited life. Sacrificial anodes are subject to ongoing corrosion, or consumption of the galvanic metal, and generally require replacement at some point depending on the extent of the corrosion.
Because corrosion of steel-reinforced concrete structures presents dangers to human life and is very costly to repair, what is needed are improved systems and methods for meeting the need to implement new anti-corrosion technologies and protect infrastructure for future generations.
It should be noted that the gaps in the graphs represent depolarizations-disconnection of the anode and cathode to determine amount of polarization and if the anode system returns to function after some time being disconnected.